1. Field of the Invention
The present invention relates to a time division multiple access (TDMA) type or a time division multiple access/time division duflex (TDMA/TDD) type mobile cummunication system.
2. Description of the Related Art
In a mobile cummunication system including a plurality of base stations, frame synchronization is needed among the base stations.
A prior art frame synchronization establishing method is carried out by a contrtol station connected to the base stations using signals of a mobile station (see JP-A-2 164140, JP-A-6-338847 and JP-A-7-154849). This will be explained later in detail.
In the above-described prior art frame synchronization method, however, since the distance between each of the base stations and the control station is not equal, the synchronization signals for the base stations are not accurate in view of the frame synchronization. Therefore, each of the base stations includes means for calculating a difference in time between a signal transmitted from the mobile station and a signal which is also the transmitted signal of the mobile station received by the other base station, and means for adjusting the synchronization signals in accordance with the difference in time. In addition, if an additional base station is provided, the establishment of frame synchronization needs to be carried out for the entire system including the additional base station, which is a problem in practice.
It is an object of the present invention to provide a mobile communication system capable of establishing frame synchronization among base stations, without a special control unit for establishing frame synchronization.
Another object is to carry out frame synchronization without changing the entire system, even when an additional base station is provided.
According to the present invention, in a time divisional multiple access type mobile communication system including a plurality of base stations, each of the base stations receives broadcast signals from the other base stations, and determines a broadcast transmission timing in synchronization with and different from timings of the received broadcast signals. Then, a broadcast signal is transmitted at the broadcast transmission timing.